In congressional testimony on Monday, Richard W. Murphy, the new assistant secretary of state for Near Eastern affairs, put the emphasis on political solutions to Lebanon's problems. Mr. Murphy, a former ambassador to Saudi Arabia , was mildly optimistic, pointing to the agenda adopted by Lebanese factional leaders at the recent Geneva talks as a ''positive beginning.''

Syria has been placing obstacles in the way of a political solution, Murphy said, but he argued that it would not be necessary to go to war with Syria to get it to use its influence to end the Lebanon conflict.

State Department officials had earlier argued that because the truck driver who attacked the marines in Beirut on Oct. 23 must have come from behind Syrian lines, the Syrians should be held at least partly responsible for the attack. Officials indicated that circumstantial evidence pointed to an Iranian-supported Shiite faction as the group that carried out the attack.

But a State Department official indicated on Tuesday that there were now several reasons for backing away from the threat of retaliation for the Beirut attack, including a need not to disrupt the Lebanese reconciliation talks as well as what might be called the European factor.

It is argued that any US attack on a Shiite Muslim group in Lebanon could complicate Lebanese President Amin Gemayel's efforts to reach agreements with Shiite factions on the future of the Middle East nation. Reagan administration officials are also conscious of a need to take West European opinion into account. A retaliatory strike in Lebanon that misfired or entailed civilian casualties might provide ammunition to critics of the new US nuclear missile deployments in Europe. The recent US invasion of Grenada has provoked charges in Europe that President Reagan is ''trigger happy.''

Another reason for playing down the threat of American retaliation in Lebanon seems to be US Defense Department reluctance to get more deeply involved in that country's conflict.

In an appearance before the House subcommittee on Europe and the Middle East on Monday, Air Force Maj. Gen. Edward L. Tixier, the deputy assistant secretary of defense for Near Eastern affairs, said in response to a question concerning the attack on the marines: ''We are not very good retaliators. We're looking for a smoking gun, and I don't care what the newspapers say, I don't know personally that we are that sure as to who the perpetrators of that deed are. We have some pretty good ideas, but I doubt whether anybody could tell you with the kind of guarantees that we would need to take retaliatory action. . . .''

None of this means that the Reagan administration is ruling out retaliation. Assistant Secretary Murphy said that the administration was continuing ''an intensified intelligence effort'' to identify the perpetrators of the truck-bomb attack, which killed 239 US marines.

But the administration clearly sees no benefit to be gained from further public discussion of the subject, or from making what sound like threats and then not following up. In an appearance on the NBC television program ''Today'' Monday, Secretary of State George P. Shultz said on the subject of retaliation that the administration had decided that ''we are not going to discuss that subject any more.''

Another State Department official said the administration was trying to look at the broader problem of which nations were sponsoring terrorists rather than responding to each and every terrorist act with specific retaliation against those terrorists. ''It doesn't do much good just to go after the thugs,'' he said.

Meanwhile, a widespread feeling persists in Congress that US military officers neglected to protect the US marines in Beirut from the kind of attack that occurred. Military officers have tended to say that there was no precise intelligence warning of a possible attack of this nature and that the truck attack that did occur was unprecedented.

Addressing General Tixier at his subcommittee's hearing on Monday, Rep. Lee H. Hamilton (D) of Indiana declared that there had been ''about a hundred intelligence warnings'' of a possible car-bomb attack. ''To say that a truck makes it an entirely different situation doesn't wash with me,'' he said.